Optical recording serves an important role in information storage. Information density is many times greater for optical recording than comparable magnetic recording and offers longer life and higher speed for reading and writing information. It also has the potential for decreased cost. Presently, however, the average access speed of optical disk drives are low and the cost high in comparison to magnetic media, due largely to the bulk and high cost of the optical read/write head.
Conventional optical read/write heads typically contain a number of glass lenses, a semiconductor laser whose output beam is elliptical in cross section, and, frequently, beam shaping elements. The read/write heads are very complex, heavy, and relatively slow to position. The glass lenses afford high-quality imaging, however, by accurately shaping and collimating the laser beam. Further, the glass lenses provide a good wave front for the beam, and remain relatively insensitive to changes in wavelength of the beam.
Holograms have been used to replace one or more optical elements in read/write heads. In theory, all lenses could be replaced by a single hologram. However, holograms are extremely sensitive to wavelength shifts and use of a single hologram is therefore impractical.
One system utilizes two holograms to replace several optical elements. The holograms are parallel to each other and are arranged to diffract light by opposing orders, e.g., first by -1.0 order and then by +1.0 order. Certain deviations such as wavelength shifts are largely cancelled by use of opposing diffraction orders. However, this system does not correct the beam shape. Therefore, beam shaping optics, typically a pair of prisms, must be added to the system to shape the beam. This is very difficult to accomplish while maintaining compactness. Beam shaping is important because masking the elliptical portion of the beam wastes a great deal of radiation. Masking an elliptical beam results in the loss of a great deal of beam power, which loss is acceptable for audio compact disk applications but unacceptable for writing optical disks utilized in computer applications.